Process for the extraction of relatively pure thorium



Oct. 3, 1961 ERVIN, JR., ET AL PROCESS FOR THE EXTRACTION OF RELATIVELY PURE THORIUM Filed Aug. 4, 1959 INVENTORS. GUY ERV/N JR HEE KTF GUELTZ 5 g g 751.

ATTORNEY 3.002.906 PROCESS FOR THE EXTRACTION OF RELA- TIVELY PURE THORIUM Guy Ervin, Jr., Encino, Calif., and Herbert F. G. Ueltz, Shrewsbury, Mass., assignors to Norton Company, Worcester, Mass., a corporation of Massachusetts Filed Aug. 4, 1959, Ser. No. 831,606 8 Claims. (Cl. 204-64) The invention relates to the extraction of relatively pure thorium. This application is a continuation-in-part of our copending application Serial #356,421, filed on May 21, 1953, now abandoned.

One object of the invention is to provide a thoroughly practical and commercial process for the extraction of this metal which process can be operated at relatively low cost.

Another object of the invention is to provide a process of the nature indicated utilizing a simple apparatus which is quite safe to operate. Another object of the invention is to provide a process of the nature indicated in which a single step only is required for transforming the metal carbide directly into this metal in such a way that there is no opportunity for contamination by oxygen or nitrogen or other undesirable impurities that are difficult to remove. Another object of the invention is to produce this metal in relatively large grain size by means of a simple process. Another object is to make malleable and ductile metal.

We have discovered a process for producing relatively pure thorium the basis for which is the electrolytic deposition of metal in an electrolytic cell having a consumable anode made of carbide of the metal. The electrolytic bath is composed of fused salt which is halide of metal selected from the group consisting of the alkali metals and alkaline earth metals including magnesium and mixtures of such halides. Also we prefer to provide a small amount of double halide of alkali metal and thorium. This accelerates the process and produces purer metal. It makes larger crystals. The fluorides are the most practical of the halides which are double salts of alkali metal and thorium and potassium is the most practical of the alkali metals. Thus we prefer potassium thorium fluoride, KThF to accelerate the process. The melting points of the chlorides of the alkali and alkaline earth metals are shown in the following table.

Table] Melting point, degrees centigrade Chloride:

Sodium chloride, NaCl 804 Potassium chloride, KCl 776 Lithium chloride, LiCl 613 Rubidium chloride, RbCl 715 Cesium chloride, CsCl 646 Magnesium chloride, MgCl 712 Calcium chloride, CaCl 772 Strontium chloride, SrCl 873 Barium chloride, BaCl 962 While we have used a eutectic mixture of 40 mols of potassium chloride and 60 mols of lithium chloride, having a melting point of about 350 C., we later found that the most satisfactory salt for the bath is common salt NaCl and of course this is the cheapest of all salts. In addition it is easily obtained in any anhydrous condition and its melting point is low enough in view of the fact that we have found that for the best results the temperature of the bath should be over 800 C. and we use an even higher temperature to avoid freezing of the salt by unavoidable variations in temperature. A preferred temperature, 900 C., is still low enough for an economical operation, all things considered.

nited States Patent ICE For purity of product, thorium is obviously the ideal metal for the cathode. But as this is radio-active and has to be handled a lot, we prefer to use another metal. Chromium plated stainless steel is both inexpensive and very satisfactory and is our choice, but many other metals could be used such as the chemically highly inert zirconium with its usual small hafnium content and as this will last ahnost indefinitely, the higher cost is not objectionable.

One apparatus, and the best one now known to us, in which the process of this invention can be carried out is illustrated in the drawing. A refractory box 1 consisting of a sheet steel cylinder 2, to the bottom of which is welded a bottom plate 3 and having a top plate 4 secured thereto by bolts 5, is filled with refractory brick. The box 1 is shown as supported by legs 8. Through a space 10 in the brick extend resistor bars 11 made of silicon carbide of a type now well known, these bars having so-called cold ends 12 as such bars practically always do. The cold ends 12 extend through alumina sleeves 15 that extend through the top plate 4- and the brick to receive the upper cold ends 12 and through the brick and the bottom plate 3 to receive the lower cold ends 12. The lower cold ends 12 are upported by refractory blocks 16 which rest upon the lower horizontal portions of Z-shaped irons 17 the upper horizontal portions of which are welded to the bottom plate 3. Electrical connections are made to the cold ends 12, but these are well known and are not shown. By energizing the bars 11 the temperature in the cell 20 can be brought to the desired level.

The cell 20 is made of steel. It has a hollow flange '21 through which cooling water is pumped by means of connection 22 and 23. It is bolted by means of bolts 24 to a head plate 25 having a hollow upward extension 26 through which Water is pumped by means of connections 27 and 28. The head plate 25 is sealed to the flange 21 by means of a ring 30 between these parts. The ring 30 is made of chlorinated butadiene.

The extension 26 has a flange which is bolted by means of bolts 36 to a flange 37 on the bottom of a pipe shaped valve body 40 transversed by a vacuum seal valve apparatus 41 which can be operated to seal off the space below it. This valve apparatus 41 is not shown in detail as it belongs in another art and any good one can be used.

Extending upwardly from the valve body 40 is a water cooled pipe 50. This is provided to allow the top of the apparatus to become relatively cool. This pipe has a bottom flange 51 and a top flange 52, and from the bottom of the former to the top of the latter the pipe 50 is two feet high. The flange 51 is bolted to an upper flange 53 provided on the top of the valve body 40 by means of bolts 55. The flange 52 is bolted to a plate 57 by means of bolts 58. The plate 57 has a central hole 60 and above this central hole 60 is a rubber sealing tube 61 the lower part of which is reinforced with a steel sleeve 62. The rubber sealing tube 61 is held down onto the plate 57 by means of a laminated cloth and phenolic resin plate 65 having a hole 66 therethrough, hold down bolts 67 extending between the plate 57 and the plate 65 being provided to hold these plates together.

The water cooled pipe 50 is cooled by a water chamber 70 welded thereto and connections 71 and 72 to circulate the water. A gasket 73 is provided between the flanges 57 and 53 and a sealing ring 74 is provided between the flanges 57 and 52, both of these being made of chlorinated butadiene. It is important to keep the system free of air, that is to exhaust the air before starting the electrolysis and to remove any contaminating atmosphere which may be generated during the electrolysis. To that end we pump through the system argon or other inert gas by way preferably of an upper pipe exhausting the gas through a lower pipe 81, the former for example extending into the top of the pipe 50 and the latter into the flange 25 and connected to a bore 82 extending to the inside of the extension 26. We find it is preferable to have the argon entrance above the argon exit to drive salt vapor downwardly to keep it from plugging the upper part of the apparatus. The system should be flushed with argon before starting electrolysis desirably for about twenty-four hours. Argon is pumped all of the time during electrolysis (but could be interrupted for short periods). In an apparatus of this size a flow of argon of two cubic feet per hour is satisfactory.

The steel cell 20 (an ordinary low carbon steel was used) was 5% inches inside diameter. The extension 26, made of the same steel, had an inside diameter of 3% inches and so did the valve body 40 and the pipe 50. All of these parts were made of the same steel except the body 40 which was made of aluminum. Dimensions of the apparatus not mentioned can be calculated closely by sealing the drawing relative to a dimension given. The cell 20 was Nichrome plated on the outside, by flame spraying.

Fitted into the cell 20 is a graphite crucible 90 and the drawing sufiiciently shows its shape and position. Inside of the graphite crucible 90 is a long sleeve made up of a series of anode rings 100 of metal carbide bonded with pitch in the manner to be particularly described.

A long rod shaped cathode 101 extends in an axial position relative to the cell 20 the crucible 90 and the sleeve 100, vertically from close to the bottom of the anode 100 through the extension 26, through the valve body 40, through the valve mechanism 41 when the valve thereof is open, through the pipe 50, through the hole 60, through the rubber sealing tube 61 and through the hole 66, projecting a slight distance above the plate 65. There it is connected by a clamp to the negative side of a source of direct current electrical energy as indicated by the negative sign above its top. The cell 20 and therefore also, through the crucible 90, the anode 100 is connected by electrical connections to the other side of the circuit which is therefore a source of positive electricity as indicated by a positive sign close to the bottom of the bolt 24 that is shown; a convenient place to make the connection. But any way of connecting the anode 100 to the positive side of the source is satisfactory.

The cathode is withdrawn from time to time to collect metal deposited thereon. To do this it is first drawn upwardly through the sealing tube 61 until its bottom has cleared the valve mechanism 41. Then the valve is closed. After an interval of time usually about an hour to allow the cathode 101 where the metal has collected thereon and said metal to cool down enough to avoid reaction with the air, the plate 65 is unbolted and lifted up and off the cathode 101, and then the cathode 101 with the deposit of metal is entirely removed from the system, and the metal is scraped off and collected for further processing which need not be described herein. Briefly such processing involves dissolving off the salt clinging to the metal, pressing the sponge metal so clean of salt, melting it in a vacuum and casting ingots, or, instead of melting and casting the metal, it can be pressed and sintered to form articles.

While the bottom of the cathode 101 and the metal thereon is cooling in the valve body 40 and pipe 50, argon or other inert gas is pumped from the pipe 80 to an exhaust pipe 105 having a valve 106 so that it can be opened at this time and closed when the cathode 101 is down and the process is operating.

Twenty pounds of thorium carbide, ThC are pulverized by ball milling dry in a steel ball mill so that the material will pass through a 100 mesh screen (Tyler series). The pulverized ThC is acid washed by placing it in a 40 gallon stoneware crock. Ten liters of a solution of 5% by volume of commercial concentrated H 80 and 95% by volume of distilled water is placed in the crock with the T110 A copper steam coil and an agitator with a rubber propellor are placed in the mixture and steam at about 10 pounds pressure is turned on and passed through the coil to heat the mixture. After the reaction has subsided or in about 10 hours the coil is removed and the ThC is allowed to settle. The supernatant liquor is eliminated after 24 hours by siphoning and distilled water is added. The mixture is rinsed in this manner 6 times. The final rinse water is siphoned off and the sludge is dried in an enameled basin in an air circulating oven at 200 F.

A mixture consisting of 5% by weight hard pitch and by weight ThC powder is made by blending 272 grams of pulverized hard pitch (200 mesh, MP. 285 to 315 F.) with 5160 grams of acid treated and dried ThC powder in a sealed fiber carton and rolling on a roller mill for two hours. Six rings, 4% OD. x 3%" ID. x 2" high are pressed from the mixture by conventional cold pressing techniques in a steel mold at 3 tons per square inch. Each ring Weighs approximately 900 grams and the density is approximately 5.3 g./cc.

The six rings are stacked in the graphite crucible 90 and placed in the cell 20. The cell is placed in the furnace. Argon gas is fed in the gas inlet. The pitch volatiles are baked out and are carried out the gas outlet by argon gas and trapped in a steel condenser. The temperature is raised in 200 C. increments to 1000 C. in 5 hours and the rings are baked at C. for five hours. After cool ing, the head 26 is removed and pitch volatile greasy condensate is scraped from the cool surfaces inside the cell, which are then cleaned with solvent such as acetone.

The pitch loses 52% by weight due to baking out the volatiles and leaves 48% carbon as the bond. The weight of the six rings after baking is approximately 5300 grams. The height of the lining is 12 inches. The density of the carbon bonded ThC is approximately 5.2 g./cc. The volume of the pores is approximately 34%. The capacity of the lined crucible is 2.2 liters. Approximately 4600 grams of thorium are available for electrolytic extraction.

The cavity of the crucible is filled with 3860 grams of electrolyte. The electrolyte is made up of 3677 grams of C.P. sodium chloride and 183 grams of vacuum dried ThC This quantity of electrolyte will give a molten salt bath depth of about 12 inches at operating temperature, with a soluble thorium concentration of 3 The temperature is turned to C. without water flowing through the cooling chambers. The vacuum pump and a dry ice trap are connected and the system is evacuated. The electrolyte and interior surfaces of the system are vacuum dried until ice formation on the dry ice trap ceases. Purified dried argon gas is allowed to fill the system. Evacuation and flushing with argon is done three times. The leak rate is measured as the mass of the air that leaks into the apparatus per minute, and is mathematically proportional to the micron pressure change per minute multiplied by the volume of the apparatus in liters, and thus expressed as micron-liters per minute. When the leak rate is below 200 micron liter per minute, the cell is tight enough for the electrolysis.

The cooling water is turned on, the bars 11 are energized and the temperature is raised to 900 C. A chromium plated stainless steel cathode is lowered into the electrolyte, while impressing a small voltage upon it, until the lower end of the cathode is two inches above the bottom of the crucible. The DC. rectifier is turned to 200 amperes, a voltage in the range of from about 5 to 10 volts being required. After one hour, the direct current is shut off and the cathode and deposit are withdrawn into cooling chamber 40. Valve 41 is closed, the cathode with adhering deposit is allowed to cool and then is removed from the chamber 40.

The deposit with entrapped salt is chipped from the cathode and crushed and leached with distilled water until all traces of electrolyte are gone. The metal powder is compacted by pressing in a steel mold and melted into a thorium metal ingot in an atmosphere of pure argon in a water-cooled arc furnace.

The best mode of the invention known to us is the example given. The apparatus described is the best now known to us and the principal way to improve it is to make it bigger.

It will thus be seen that there has been provided by this invention a process for the extraction of relatively pure thorium in which the various objects hereinabove set forth together with many thoroughly practical advantages are successfully achieved. As many possible embodiments may be made of the above invention and as many changes might be made in the embodiments above set forth, it is to be understood that all matter hereinbefore set forth or shown in the accompanying drawing is to be interpreted as illustrative and not in a limiting sense.

We claim:

1. Process for the preparation of thorium which comprises passing a direct electric current through a cell having a solid anode and a solid cathode in a direct curthe positive side of the electric circuit and collecting the thorium metal electrolytically liberated at the cell cathode.

2. Process according to claim 1 in which the anode is made of said carbide.

3. Process according to claim 2 in which the major portion of the fused halide is chloride.

4. Process according to claim 3 in which the major portion of the fused chloride is alkali metal chloride.

5. Process according to claim 1 in which the major portion of the fused halide is chloride.

6. Process according to claim 5 in which the major portion of the fused chloride is alkali metal chloride.

7. Process according to claim 1 in which the major portion of the fused halide is alkali metal halide.

8. Process according to claim 7 in which the anode is made of said carbide.

References Cited in the file of this patent UNITED STATES PATENTS rent electric circuit, the electrolyte in said cell consisting, 5,025 Briggs et 1 8, 1 1 apart from any thorium halide content, essentially of fused halide of metal selected from the group consisting FOREIGN PATENTS of alkali metals and alkaline earth metals including mag- 334,475 Germany Mar. 14, 1921 nesium and mixtures of such halides, said cell containing 137,626 Sweden Oct. 14, 1952 thorium carbide in said halide electrically connected to Patent N0. 3,002,906

Guy Ervin, Jr

October 3, 1961 et a1.

It is hereby certified that error appears in the above numbered patent requiring correction and that the said Letters Patent should read as corrected below.

Column 4, line 27, for C." read 1O00 C.

(SEAL) Attestr DAVID L. LADD Commissioner of Patents ERNEST W. SWIDER Attesting Officer UNITED STATES PATENT. OFFICE CERTIFICATE OF CORRECTION Patent N0. 3,002,906 October 3, 1961 Guy Ervin, Jr. et a1.

It is hereby certified that error appears in the above numbered patent requiring correction and that the said Letters Patent shouldread as corrected below.

Signed and sealed this 11th day of September 1962.

(SEAL) Attestz' DAVID L. LADD Commissioner of Patents ERNEST W. SWIDER Attesting Officer Patent No. 3,002,906

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION October 3 1961 Guy Ervin, Jr. et al.

It is hereby certified that error appears in the above numbered patent requiring correction and that the said Letters Patent shouldread as corrected below.

Column 4,

lin e 27, for "100C." read -'-1000 co SEA L) Attest:

ERNEST W. SWIDER Attesting Officer DAVID L. LADD Commissioner of Patents 

1. PROCESS FOR THE PREPARATION OF THORIUM WHICH COMPRISES PASSING A DIRECT ELECTRIC CURRENT THROUGH A CELL HAVING A SOLID ANODE AND A SOLID CATHODE IN A DIRECT CURAPART FROM ANY THORIUM HALIDE CONTENT, ESSENTIALLY OF FUSED HALIDE OF METAL SELECTED FROM THE GROUP CONSISTING OF ALKALI METALS AND ALKALINE EARTH METALS INCLUDING MAGNESIUM AND MIXTURES OF SUCH HALIDES, SAID CELL CONTAINING THORIUM CARBIDE IN SAID HALIDE ELECTRICALLY CONNECTED TO THE POSITIVE SIDE OF THE ELECTRIC CIRCUIT AND COLLECTING THE THORIUM METAL ELECTROLYTICALLY LIBERATED AT THE CELL CATHODE. 